STIHL MS 391 Service Manual

Overview and Safety Information

The STIHL MS 391 service manual section begins with the explicit purpose of providing technicians with clear, step-by-step guidance for inspection, maintenance, troubleshooting, and repair of the MS 391 chainsaw. This portion defines the intended use of the manual as a practical reference for field technicians, shop mechanics, and service center staff who perform routine servicing as well as more complex repairs. It emphasizes consistent procedures, torque specifications, and safety precautions to ensure reliable operation and user safety. By outlining the scope and target audience, this section helps technicians quickly determine which information is relevant to the specific service task at hand and reduces the risk of applying incorrect procedures.

Safety guidelines and personal protective equipment (PPE) are presented as a foundational element of all service activities. The manual outlines mandatory PPE such as cut-resistant gloves, eye and hearing protection, and sturdy, non-slip footwear, along with optional but recommended items like a face shield or arc flash-rated eyewear for certain servicing scenarios. It also stresses the importance of disconnecting the spark plug cap, relieving fuel system pressure, and securing the bar and chain to prevent accidental movement during maintenance. Additional safety notes cover proper handling of sharp components, hot surfaces, and stored energy in the compression release system, ensuring technicians minimize the risk of lacerations, burns, or acoustic damage. The section reinforces the need for a clean, organized workspace, adequate ventilation when testing, and adherence to manufacturer torque specifications to maintain machine integrity and operator safety.

The applicable models and serial number range are clearly defined to prevent the use of the manual on incompatible equipment. The MS 391 is identified by its specific production year, engine displacement, bar length, and other identifying features that determine which service procedures are valid. The section includes a serial number mapping that indicates during which production runs certain components were updated or replaced, ensuring technicians apply the correct parts and service instructions. It also provides guidance on how to verify model and serial numbers on the unit before beginning work, reducing the likelihood of applying outdated procedures or incompatible parts. This careful model and serial alignment helps maintain warranty eligibility, ensures correct parts availability, and supports accurate diagnostic results during service operations.

Technical Specifications and Tools Required

The STIHL MS 391 is a robust forestry chainsaw designed for demanding felling, limbing, and bucking tasks. Its engine specifications include a powerful two-stroke gasoline-fueled powerhead that delivers reliable torque across a wide RPM range, optimized for sustained cutting performance in tough wood species. When reviewing the engine, pay attention to the displacement, compression, and ignition timing, as these factors influence starting behavior and overall efficiency. The model typically features a carbureted or semi-electronic fuel system depending on production year, with a tuned exhaust and lubrication system to support prolonged operation. Familiarize yourself with the recommended duty cycle and operating temperatures to prevent premature wear and to maximize service life. Regular inspection of the air intake, cooling fins, and fuel lines helps maintain consistent performance and reduces the risk of overheating during extended use.

Bar and chain information for the MS 391 focuses on optimal cutting performance and safety. The standard bar length is chosen to balance maneuverability with cutting depth, while the chain pitch, gauge, and drive links determine how smoothly the chain tracks along the guide bar. Ensure the chain is properly tensioned—not too tight to hinder lubrication and not too loose to cause derailment during high-speed cutting. Inspect the drive sprocket for wear and verify the chain brake function engages promptly when activated. Regularly inspect the bumper spike and tip guard for cracks or deformation, as these components contribute to stability and kickback control. Keep the chain sharpening consistent with manufacturer recommendations to maintain predictable cutting behavior and to reduce fuel consumption caused by dull teeth.

Lubrication and fuel requirements are critical for reliable operation and longevity of the MS 391. Use the correct two-stroke engine oil-to-fuel ratio as specified by STIHL for your altitude and temperature range, and mix thoroughly to prevent varnish and gum buildup in the carburetor. The bar oil reservoir should be filled with STIHL bar and chain oil or an equivalent high-quality lubricant that provides adequate viscosity under expected operating conditions. Monitor oil flow and adjust the oiler setting to ensure proper lubrication of the chain in the bar groove, especially when cutting in dusty or dirty conditions. Temperature and age of fuel can impact starting reliability, so always use fresh, stabilized gasoline from a reputable source and follow segregation practices to avoid cross-contamination with stored fuel. Regular draining and cleaning of the fuel and lubrication systems help prevent clogs and maintain smooth operation during long work sessions.

Tool list for maintenance and servicing includes a comprehensive set of wrenches, screwdrivers, and diagnostic tools tailored to the MS 391’s fasteners and components. A spark plug wrench and gap gauge are essential for regular ignition checks, while a feeler gauge helps verify timing adjustments and carburetor settings. A chain file or round file kit with appropriate gauges is required for routine chain sharpening, along with a strop or flat file for edge finishing to preserve cutting efficiency. A depth gauge ruler aids in maintaining correct chain tolerances, and a torque wrench ensures that bar bolts and sprocket fasteners are secured to manufacturer specifications. For safety and efficiency, include a pair of locking pliers, a small magnifying loupe for visual inspection, a compression tester if available from the manufacturer, and personal protective equipment such as gloves, eye protection, and hearing protection. A dedicated cleaning brush and compressed air can simplify removal of sawdust and debris from cooling fins, air intake, and fuel passages between service intervals to sustain consistent performance.

Initial Inspection and Troubleshooting Guidance

The initial inspection focuses on establishing a baseline condition of the STIHL MS 391 before attempting any start or operation. Begin by performing a thorough visual assessment of the exterior for any signs of damage, loosened fasteners, or fuel leaks. Check the air filter for dirt and obstruction, and inspect the spark plug for fouling or wear, ensuring the electrode gap is within the manufacturer’s specification. Examine the recoil starter, fuel lines, and fuel tank for cracks or leaks, and verify that the chain brake engages and disengages smoothly. A clean, dry environment is essential; ensure the bar and chain are properly lubricated and free of excessive tension before proceeding with any tests. Document any abnormal findings and address them in the sequence of checks to prevent misdiagnosis during starting attempts.

During the initial inspection, also confirm that the correct fuel mixture and oil are being used, as incorrect ratios can impair starting, idling, and overall performance. Inspect the carburetor for signs of varnish or buildup, and confirm that the air intake is clear from obstructions. Verify the muffler and spark arrestor screen for carbon buildup, which can impede exhaust flow and cause overheating. Ensure that the chain is sharp and properly aligned with the guide bar, and that the bar groove is not chipped or fouled by dirt. A systematic sweep of these components helps identify root causes that might masquerade as more serious failures later in the process.

When conducting starting and idling troubleshooting, ensure the unit is on a stable, level surface with the chain brake engaged. Begin with fresh fuel and a properly charged spark plug; if applicable, replace the spark plug to rule out ignition issues. Check the choke and throttle lockout functions to confirm they operate smoothly, and verify that the primer bulb, if present, delivers a consistent spray of fuel without leaks. For cold starts, follow the correct sequence of pull-cord action and choke use; for warm starts, minimize choke engagement and observe whether the idle stabilizes. If starting issues persist, perform a compression check to determine if the engine rings, valves, or cylinder have degraded performance characteristics, and compare readings against the manufacturer’s specifications.

In performance and diagnostic procedures, establish baseline operating conditions by warming the engine to normal operating temperature and verifying that the saw reaches a stable idling speed without fluctuation. Listen for unusual noises such as knocking, pinging, or grinding, which can indicate internal wear or loose components. Observe exhaust color; blue or black smoke can signal incorrect fuel mixture or worn engine parts. Use a diagnostic approach that includes checking the air-fuel ratio, carburetor adjustments, and fuel delivery pressures; adjust as necessary within spec to achieve smooth acceleration and consistent power delivery. If performance remains abnormal, inspect the cooling fins and flywheel for debris that could cause overheating or timing irregularities, and confirm that the exhaust system is free of obstructions. A methodical diagnostic routine helps pinpoint whether the issue lies in ignition, fuel delivery, compression, or mechanical wear, guiding safe and effective repair decisions.

The STIHL MS 391 chainsaw requires careful attention when removing the bar and chain to prevent injury and damage to components. Begin by ensuring the unit is powered off, the spark plug ground is disconnected, and the chain brake is engaged. Remove the bar nuts and drive sprocket cover, keeping track of any washers or spacers that may be present. Gently lift the bar away from the saw, guiding the chain off the bar and ensuring the drive links do not slip or bend. When storing or transporting the components, place the chain on a clean, flat surface and avoid contact with sharp cutting edges to prevent accidental injury.

Accessing the crankcase and filter chamber involves removing covers and securing the internal area for inspection. After removing the drive sprocket cover, inspect the bar groove and drive sprocket for wear, and remove the clutch drum if required using appropriate tools and torque to avoid damage. Once the crankcase cover is exposed, disconnect any fuel lines or carburetor linkage only if necessary and documented by service procedures. Clean the exterior surfaces to prevent debris from contaminating the filter chamber, then carefully detach the air filter housing to reveal the filter chamber and its contents. Handle the crankcase components with care to avoid scratches or nicks that could affect seals and tolerances during reassembly.

The reassembly process and torques checklist should be approached systematically to ensure proper function and safety. Start by installing new or inspected gaskets and seals, then reattach the crankcase cover, ensuring it seats fully and the gasket is not pinched. Reassemble the filter chamber with a new or cleaned air filter, confirming a proper seal around the housing to prevent unfiltered air from entering the intake system. When reinstalling the bar and chain, align the drive links with the sprocket, thread the bar nuts to the specified tightness, and verify the chain tension per the manufacturer’s specification. Finally, reference the torques checklist to confirm the correct fastener values across the assembly, and perform a functional test with the chain brake engaged to ensure safe operation before returning the saw to service.

Carburetor and Fuel System Servicing

The STIHL MS 391 carburetor and fuel system play a critical role in delivering the correct air-fuel mixture for optimal engine performance. Proper identification of the carburetor model and its mounting arrangement is essential before disassembly. Begin by ensuring the saw is cool, disconnect the spark plug boot, and drain any residual fuel from the tank. Visual inspection should focus on hoses, clamps, and the mounting hardware to determine access points and any pending wear that could affect service procedures. Having the correct service tools, including metric wrenches, small pliers, and a flat-head screwdriver, helps reduce the risk of damage during disassembly. When accessing the carburetor, take careful note of any alignment marks and gasket positions to ensure accurate reassembly later. Maintaining a clean work surface and using a parts tray prevents small components from being lost and preserves the integrity of the repair process.

Access to the carburetor involves removing the air filter cover, air filter, and fuel line connections. Before removing the carburetor, document the hose routing and vacuum control connections to aid reassembly. Carefully detach the intake boots and ensure any isolation springs or detents are preserved. Inspect the carburetor mounting studs for wear or damage and replace as necessary to guarantee a secure, leak-free installation. When removing the carburetor, avoid bending the throttle linkage or fuel inlet hose, as distortions can cause unintended high idle or fuel leaks. After removal, examine the carburetor body for signs of varnish, gum, or dirt buildup, which can impede metering and performance. Cleanliness at this stage is paramount to achieving reliable operation after reassembly.

Adjustments and cleaning procedures require precision and adherence to the manufacturer's specifications. Start with the idle speed and mixture screws, ensuring the engine exhibits a stable idle without stalling and maintains smooth acceleration. Use the appropriate tool to turn the screws in small increments, testing engine response after each adjustment. Disassembly of the carburetor is sometimes necessary to address varnish buildup, which can alter fuel metering and air passage sizes. When cleaning, employ non-aggressive carburetor cleaners and a soft brush to remove residue without damaging orifices and jets. After cleaning, inspect all gaskets, diaphragms, and springs for wear and replace any compromised components to prevent air leaks or improper metering. Reassemble with careful alignment to preserve the integrity of the throttle and choke linkage, and verify that all connections move freely without binding. Finally, perform a test run under load to confirm that the adjustments deliver stable operation across the full throttle range and that no leaks or abnormal noises are present.

Fuel system inspection and seal replacement focus on ensuring the fuel path is free of obstructions and that seals maintain proper isolation between the carburetor, fuel tank, and intake. Inspect the fuel lines for cracking, stiffness, or hardening, replacing any aged hoses to prevent leaks or fuel seepage. Check the fuel filter, which should be clean and free of debris; replace it if contamination is detected or if flow appears restricted. Inspect the fuel pickup and lines within the tank for cracks or misrouting that could cause air ingestion or inconsistent fuel supply. If the gaskets or o-rings show signs of deterioration, replace them with OEM specifications to maintain proper seal integrity. After inspecting and replacing components, purge the fuel system of any air by gradually priming the system and confirming that fuel flows smoothly to the carburetor without pulling air pockets. Finally, perform a functional test with the cover removed to observe the fuel supply during cranking and running, ensuring there are no leaks and that the system maintains stable pressure throughout operation.

The ignition system of the STIHL MS 391 is a critical component that ensures reliable starting and consistent engine performance under varying loads. Regular inspection of the spark plug helps prevent misfiring, hard starting, and loss of power. When examining the spark plug, check for proper gap, electrode wear, and soot buildup, and replace the plug if the insulator is cracked or if the ground electrode shows excessive wear. Use the correct heat range specified by STIHL and ensure the spark plug torque aligns with the manufacturer’s specifications to maintain a proper seal and electrical contact. After installation, perform a quick cranking test to verify that a bright, blue spark is produced under compression; a weak or intermittent spark can indicate ignition coil or wiring issues that require further diagnosis. Keep the spark plug clean and gapped to the recommended specification to preserve efficient combustion and fueling.

Testing the ignition coil involves verifying that the primary and secondary windings deliver adequate voltage and resistance within published tolerances. Begin with a visual inspection of the coil housing and connections for cracks, corrosion, or loose leads. Use a digital multimeter to measure the primary resistance, then compare the reading to the service data for your model; an out-of-range value suggests a faulty coil. Next, check the secondary resistance by measuring from the spark plug boot to ground, ensuring the value matches the specified range. If the coil fails these checks, it should be replaced with a OEM part to avoid compatibility issues and ensure reliable timing. After replacement, inspect all wiring harnesses for signs of abrasion or heat damage, and secure connectors to prevent intermittent failures during operation.

Timing verification procedures are essential to ensure the engine delivers proper power, throttle response, and fuel efficiency. Begin by confirming the flywheel timing mark aligns with the corresponding reference on the crankcase, and inspect the nut torque on the flywheel retaining hardware to prevent timing drift. Remove the spark plug to relieve compression, then rotate the engine by hand and observe the position of the timing indicators, ensuring they align within spec at top dead center. If timing is off, consult service data for the exact clearance values and adjust using the appropriate advance/retard mechanism or timing shim as required by the MS 391 design. After any adjustment, reinstall the spark plug, reconnect the ignition switch, and perform a safe starting test to verify smooth idle, consistent acceleration, and absence of misfires. Document all measurements and replacement parts used to maintain a complete service history for future diagnostics.

Air System and Filtration Maintenance

The air system on the STIHL MS 391 is critical for optimal engine performance and longevity. Begin by inspecting the air filter for dirt, debris, and signs of wear. A clean filter promotes efficient airflow to the carburetor, which helps maintain correct fuel metering and combustion. If the exterior of the filter is visibly soiled or the filter media is torn or water-damaged, replace it with an OEM part to ensure proper sealing and filtration. When reinstalling, ensure the filter is seated firmly in place and all clips or fasteners are engaged to prevent unfiltered air from bypassing the filtration path.

Carburetor air intake sealing is essential to prevent unmetered air from entering the intake tract, which can cause lean running conditions and hot spots that may harm the engine. Inspect all seals, gaskets, and intake boots for cracks, tears, or deformation. If you locate any compromised components, replace them using the correct STIHL seals and boot parts. After replacement, verify that the intake path is airtight by applying a light soapy mist around joints while the engine is running and listening for air leaks or changes in idle. Address any audible leaks immediately to preserve carburetor calibration and engine performance.

Chamber cleaning and diagnostic checks involve evaluating the condition of the combustion chamber and related passages. Remove the spark plug and inspect the spark plug tip and porcelain for fouling, oil, or severe wear, which can indicate fuel mixture or ring seal problems. With the spark plug removed, perform a compression check to confirm engine health; a reading significantly lower than the manufacturer’s specification warrants further inspection of the piston, rings, and cylinder. Clean the intake passages and carburetor mounting surfaces to remove carbon buildup, using non-abrasive brushes and appropriate solvents as recommended by STIHL. Re-seat all components carefully, and re-torque fasteners according to the service data to avoid leaks and ensure stable operation under load.

Lubrication and Bar Oil System

The STIHL MS 391 uses a dedicated chain lubrication system to ensure consistent bar and chain performance under varied cutting conditions. Proper lubrication reduces friction, minimizes wear on drive components, and helps maintain efficient cutting. A well-maintained lubrication system also contributes to extending bar life and sustaining optimal engine efficiency by preventing excessive heat buildup at the cutting contact surfaces. Regular checks of all lubrication-related components should be integrated into routine maintenance to avoid unexpected downtime in the field.

Chain lubrication components include the oil reservoir, oil pump, oil lines, and the bar. The reservoir stores the bar oil and provides a steady supply to the pump. The pump and lines deliver oil to the bar nose and the upper drive module, where it is distributed across the chain. It is essential to inspect these parts for cracks, leaks, or blockages, and to verify that the oil exit at the bar nose remains clear. Contaminants in the oil can impede flow, so using clean oil and replacing the oil filter when specified will help maintain consistent lubrication performance.

Oil delivery system inspection involves verifying pump operation, checking for proper oil pressure at idle and under load, and confirming that the bar oil port at the bar nose is free of obstructions. With the bar removed, inspect the oil pump gears or eccentric regulator for wear or damage that could reduce flow. Ensure that the oil delivery lines are securely connected and free of kinks or leaks. If the system shows any signs of reduced flow, perform a systematic inspection starting from the reservoir seal, through the feed line, to the bar nose, and replace any worn or damaged components as needed.

Bar oil capacity and refill procedures require accurate knowledge of the oil capacity specified for the MS 391. Always use fresh, clean bar oil recommended by STIHL, noting that some conditions may warrant higher-viscosity oils in colder environments to maintain adequate flow. Before refilling, wipe the reservoir cap area and check for any cracks or damage to the reservoir. When refilling, do so slowly to avoid air entrapment, and reinstall the cap securely to prevent leaks. After a refill, run the engine briefly to ensure oil reaches the bar and confirm there are no leaks at the bar nose or reservoir seal. Keeping the oil at the proper level ensures reliable lubrication during operation and reduces the likelihood of overheating or increased chain wear.

Cooling System and Chip Management

The cooling system on the STIHL MS 391 is designed to maintain optimal operating temperatures under a range of cutting conditions. Begin with a thorough visual inspection of the cooling air intake and housing to ensure there are no blockages or damaged fins that could restrict airflow. Remove any debris, dust, or sap buildup using a soft brush and a low-pressure air source, taking care not to bend or deform delicate cooling fins. Verify that the air passages beneath the flywheel remain clear and that there is an unobstructed path for air to flow across the cylinder and between the cooling fins. A clean cooling path helps prevent overheating, which can shorten engine life and degrade performance over time.

Chipper and debris management play a critical role in maintaining the MS 391’s efficiency and reliability. Regularly inspect the chipper housing and surrounding areas for lodged material that could impede the exhaust flow or create hot spots. Remove accumulated chips and dust with a blunt tool or scraper, avoiding metal implements that could scratch or damage the finish. Pay particular attention to the areas around the muffler and intake because debris can accumulate and restrict air movement. Implement a routine cleaning schedule after heavy cutting sessions or when working in extremely dusty environments to sustain consistent cutting performance and reduce engine wear from heat buildup.

Safety considerations during cleaning are essential to protect both the operator and the equipment. Always disconnect the spark plug cap and allow the engine to cool before performing any cleaning or inspection procedures to prevent accidental starting. Wear cut-resistant gloves and eye protection to guard against sharp edges and flying debris. Use only approved cleaning tools and avoid blasting with high-pressure air on sensitive components, which can force debris deeper into the cooling passages. When removing debris from the cooling fins, work gently to preserve fin integrity, as bent or damaged fins compromise cooling efficiency. After maintenance, double-check that all covers are securely fastened and that there are no foreign objects left inside the housing that could interfere with air flow or cause damage during operation.

Additional preventative steps include scheduling periodic checks of the flywheel cooling fan for cracks or wear and ensuring the recoil starter assembly does not obstruct air intake. Inspect the spark arrestor screen and exhaust path for residue that may interfere with cooling and combustion efficiency. Maintain a clean environment around the saw during storage to minimize dust ingress. By integrating routine cooling and debris management into your maintenance plan, you extend the life of the engine, sustain power output, and reduce the likelihood of unexpected downtime on the job site.

Vibration, Noise, and Emission Controls

The STIHL MS 391 is designed with several integrated systems to manage vibration, noise, and emissions, ensuring operator comfort, equipment longevity, and compliance with applicable environmental standards. Proper maintenance of these systems begins with understanding how the internal components interact to dampen vibration and minimize sound without compromising performance. Regular inspection of mounting points, fasteners, and isolation bushings is essential because wear or loosening can increase reflected vibrations and noise, reducing operator control and increasing fatigue over long periods of use. When addressing vibration concerns, focus on the engine mount, handlebar assemblies, and chain brake linkage, as these are common sources of excess movement that can transfer oscillations to the user. Maintaining clean lubrication channels and ensuring that moving parts operate smoothly will also contribute to lower vibration levels during cutting and idle operations.

Noise reduction measures center on proper bar and chain maintenance, correct chain tension, and consistent bar lubrication. A sharp chain with proper tension slices through wood more cleanly and with less resistance, which in turn reduces noise generation compared with a dull or over-tightened chain. Regularly inspect the bar rails for wear, burrs, or warping, and deburr any rough edges to prevent excessive pitch and rumble during operation. Additionally, verify that the engine’s muffler and air intake are free of obstructions and that the spark arrestor, if present, is clean and correctly fastened. An exhaust system that is functioning as designed will minimize backpressure and emissions-related noise, contributing to a quieter operating environment. Always perform these checks using the recommended service intervals and torque specifications to maintain the intended acoustic profile of the unit.

Emission system servicing guidelines focus on ensuring the engine maintains proper fuel-air mixture and exhaust flow to meet environmental standards while preserving performance. Begin by confirming that the fuel is fresh and the carburetor is correctly adjusted for the prevailing altitude and temperature, as improper tuning can increase hydrocarbon output and noise. Inspect the fuel lines and primer bulb for signs of cracking or leaks, replacing any worn components to prevent vacuum loss and erratic engine behavior, which can negatively affect emissions. Check the air filter for contamination and replace it if restricted, since a clean air path supports more complete combustion and lowers unburned fuel emissions. Regularly inspect the exhaust port and muffler interface for carbon buildup, removing deposits as needed to maintain efficient exhaust flow and reduce backpressure-related noise and emissions. Adhere to STIHL-recommended maintenance intervals and use only approved components to ensure the MS 391 continues to operate within the designed emission performance envelope.

Accessories, Modifications, and aftermarket Considerations

Compatible bars and chains for the STIHL MS 391 should be selected to balance performance, durability, and safety. When choosing a replacement bar, verify the gauge, width, and mounting hole dimensions to ensure a secure fit with the drive sprocket and chain. The recommended bar lengths for the MS 391 typically range from 16 to 20 inches, depending on the type of work and operator preference. Always inspect the bar for wear or damage before installation, and replace if there are dents, cracks, or excessive wear in the rails. Use only chains that match the bar’s pitch, gauge, and drive link count to maintain proper chain tension and cutting efficiency. Regularly oil the bar’s rails and ensure the oil passage remains clear to prevent bar runout and heat buildup during extended cutting sessions.

Using aftermarket parts safely requires a careful assessment of compatibility, quality, and safety certifications. Reputable manufacturers will provide specifications that align with STIHL’s design standards, including chain pitch, gauge, and drive link counts. Avoid unbranded or questionable components that could compromise the saw’s balance, lubrication, and safety features. Aftermarket components should be installed following the manufacturer’s torque specifications and seating procedures, and always retension the chain after a brief run-in period to account for any settling. Remember that improper parts can affect performance, increase vibration, and potentially void the warranty, so prioritizing quality and compliance is essential.

Attachment installation procedures demand attention to proper alignment, secure fasteners, and correct tension. When mounting a bar and chain, ensure the bar groove is free of debris and that the chain sits properly in the groove with the drive links engaging the drive sprocket. Apply a light coating of bar and chain oil to reduce friction during initial operation, then gradually increase throttle after the chain completes a few cuts to allow the oiling system to establish full lubrication coverage. Use a calibrated tensioning method to achieve the correct tension—chain should snap back about a half inch (12 mm) from the underside of the bar with the engine off. After installation, perform a brief idle test and light cuts to verify that there is no excessive slack, binding, or abnormal wobble, and inspect for heat buildup along the bar and chain after extended use.

Maintenance Schedule and Record Keeping

Routine maintenance intervals for the STIHL MS 391 are established to balance performance, reliability, and longevity of the saw. Regularly reviewing the user manual and service guidelines helps ensure that components such as the air filter, spark plug, fuel system, and bar/chain remain in optimal condition. Establishing a default cadence based on operating hours and usage type (heavy daily logging versus occasional homeowner cuts) allows for predictable maintenance without overlooking critical tasks. When documenting maintenance, note the date, hours of operation, and any observed irregularities to build a trustworthy history that informs future service decisions. This disciplined approach reduces the risk of unexpected downtime and extends the service life of the machine.

Performance checks and documentation are essential to verify that the MS 391 delivers the expected cutting efficiency and safety. Conduct test cuts on a representative material to assess bar and chain performance, RPM behavior, and noise levels, and record the results. Include measurements of chain tension, chain lubrication status, and air filter condition as part of each check. Maintaining a detailed log of fuel quality, oil-to-fuel ratio, and line pressure (where applicable) helps diagnose performance drift and identify maintenance needs before they impact productivity. Documentation should also capture any corrective actions taken, parts replaced, and the dates of service so that future troubleshooting can leverage this historical data. A consistent format for entries ensures that information is easily searchable for future reference or when sharing with a service professional.

Troubleshooting log best practices involve capturing symptoms, probable causes, and corrective steps in a clear, chronological sequence. Start with a concise description of the issue, such as starting difficulty, stalling, loss of power, or unusual vibrations. For each symptom, list potential causes that are most likely for the STIHL MS 391 and document the tests or observations performed to rule them in or out. Record the final resolution, including details like parts replaced, adjustments made, and performance after repair. Maintaining a centralized, time-stamped log—whether in a notebook or digital instrument—facilitates trend analysis and supports warranty claims or service plan reviews. Regularly review the troubleshooting entries to identify recurring patterns that may indicate a broader issue requiring preventive action or more thorough diagnostic procedures.

Troubleshooting Reference Guide

Stihl MS 391 users often encounter a range of performance issues that can stem from fuel, air, ignition, or compression problems. A practical approach begins with a systematic check of the most common symptoms and their likely causes. By understanding these typical fault patterns, you can quickly determine whether an issue is due to routine maintenance items or something requiring deeper inspection. This guide focuses on practical diagnosis and clear corrective steps to get your chainsaw back to optimal operation while maintaining safety standards. Documenting symptoms as they occur can also help you track recurring problems and identify wear patterns over time.

Common symptoms and likely causes include hard starting or no-start conditions, rough idle, loss of power, excessive exhaust smoke, unusual noises, or chain speed irregularities. For each symptom, the likely culprits often involve spark timing, filter cleanliness, fuel quality, and carburetor calibration. Another frequent source is improper inertial vacuum leakage or degraded seals that affect air-fuel balance. Remember that repeated or combined symptoms may indicate multiple contributing factors, such as a clogged air filter paired with a worn spark plug. A careful initial inspection should always verify fuel presence, correct bar and chain lubrication, and proper carburetor adjustment before pursuing more invasive repairs.

Step by step corrective actions begin with a thorough safety check, including removing the spark plug boot and ensuring the saw is cool before any maintenance. Start with the simplest fixes: replace old or contaminated fuel, clean or replace the air filter, and inspect the spark plug for wear, fouling, or incorrect gap. If starting issues persist, inspect the fuel line for cracks or pinholes, verify the fuel has the correct octane and ethanol content, and ensure the carburetor is calibrated to the manufacturer's specifications. For power and performance concerns, clean the carburetor jets, inspect the throttle mechanism for binding, and confirm the muffler screen is not blocked. After performing these steps, always re-check compression and spark quality to confirm that a basic service has resolved the issue before moving to more advanced diagnostics.

When to seek professional service is a critical consideration, as some conditions require specialized tools and procedures. If the engine exhibits persistent loss of power after comprehensive carburetor service, abnormal engine noises, overheating, or excessive vibration, professional diagnosis is advised. Fuel system or carburetor work that involves disassembly beyond basic cleaning, or any suspected crankcase issues, should be handled by a qualified technician. If the chainsaw has sustained physical damage, a suspected timing failure, or if the bar and chain system show signs of improper alignment or wear that could compromise safety, professional servicing is essential. Always observe local regulations and Stihl service guidelines to ensure that repairs are performed to factory standards and that the warranty remains valid where applicable.

Appendix and Reference Materials

The following materials provide essential support for technicians performing maintenance and repair on the STIHL MS 391. These references are designed to supplement the main service documentation by offering detailed diagrams, parts identification, and commonly used terminology. Careful consultation of wiring diagrams and schematics can dramatically reduce diagnostic time and improve repair accuracy, especially when tracing ignition, charging, and sensor-related concerns. When in doubt, verify any schematic against the specific production date and regional variant to ensure correct interpretation of connector types and color conventions.

Wiring diagrams and schematic references form the backbone of electrical troubleshooting for the MS 391. These diagrams illustrate the routing of wires from the ignition module, kill switch, and coil through the harness to the safety interlocks and switch assemblies. They also clarify the grounding schemes and the routing of signals for the carburetor’s electronic components, if equipped with any fuel management features in later configurations. For optimal results, cross-reference color codes and connector pinouts with the actual harness assemblies found in the unit being serviced, as minor production revisions can alter terminals or harness routing. Always isolate the spark system and disable the ignition when performing persistent electrical checks to prevent accidental engine kickback or shock.

Supplier and parts list references provide authoritative sources for correct replacement components and compatible upgrades. This section typically includes part numbers, revisions, and dimensional specifications that ensure you select the right items for your specific model and production year. When ordering, confirm the saw’s serial number and the local market variation to avoid mismatches in bar studs, anti-vibration mounts, and carburetor parts. The parts list also aids in determining service intervals for consumables such as filters, gaskets, and seals, helping to plan preventive maintenance and reduce unexpected downtime. Maintain an organized record of part numbers and supplier contacts to streamline future repairs and warranty verifications.

The index of terms and glossary consolidates common terminology used throughout the service manual into a concise reference. This glossary covers engine mechanics terms, fuel system jargon, and electrical descriptors that technicians may encounter during disassembly and reassembly. Clear definitions for items like decompression releases, various springs, and gasket materials reduce ambiguity and speed up communication within a maintenance team. For non-native English speakers or technicians transitioning from other brands, the glossary also includes cross-references to equivalent terms and standard abbreviation conventions used in STIHL documentation. Regular consultation of the glossary supports consistent terminology usage across trims, ensuring precise ordering and effective troubleshooting.